MEMS pressure sensors are generally known and widely used. One type of pressure sensor is an absolute pressure sensor which includes a pressure sensing element made of silicon which is anodically bonded to a glass pedestal to form a reference vacuum. The pressure sensing element includes four piezoresistors connected into what is known as a “Wheatstone Bridge” configuration, which are used for detecting changes in pressure.
Many absolute pressure sensors are used in applications in which the sensors are exposed to a harsh media. For such applications, the front side sensing by a traditional absolute pressure sensor cannot survive in the harsh media. It requires another type of absolute pressure sensor, such as a backside absolute (BSA) pressure sensor, which is resistant to exposure to harsh media. One typical BSA pressure sensor includes a top cap to enclose a reference vacuum, and expose the backside of the sensor to media.
However, current designs for the BSA pressure sensors are subject to structural failures. The bonding between the cap and the pressure sensing element is not always robust enough to maintain the hermetic bonding between the cap and the pressure sensing element.
One type of approach to bond the cap to the pressure sensing element is to use bonding films, such as a layer of polysilicon and a layer of silicon oxide in between the cap glass and the pressure sensing element. A problem that exists with this approach is output instability due to mobile ions. The cap glass has Na+ ions which, along with contaminated ions, may migrate to the silicon oxide layer. Amorphous silicon oxide has many open channels, which allow for movement of Na+ ions. Furthermore, when in operation, there exist strong electric fields in the silicon oxide and silicon surfaces that drive the ions to move, causing a change in the output of the pressure sensor over time, or output instability. The effect of output instability is more pronounced at high temperatures.
An attempt to overcome the output instability due to the movement of mobile ions as mentioned above is to incorporate a layer of silicon nitride film (Si3N4 deposited by low pressure chemical vapor deposition technique) in between polysilicon and silicon dioxide. Silicon nitride has proven to be an efficient dielectric layer to block mobile ions from getting into the silicon oxide and silicon surface. However, the use of silicon nitride presents other issues. Si3N4 film naturally has poor adhesion to polysilicon film. This causes integrity issues resulting from weak cap bond strength due to poor adhesion between the bonding films, such as polysilicon and silicon nitride. Some mechanical or environmental disturbance may cause interfacial failure at the interface between the silicon nitride and the polysilicon, or moisture and/or air diffusing in through the interface causing a change in the vacuum level over time. Maintaining a robust silicon nitride film is important to device performance. The type of failure which results is a failure at the interface between the silicon nitride and the polysilicon during a die shear test, or interfacial failure.
Additionally, in order for the silicon nitride to provide robust passivation, the silicon nitride must have a thickness of greater than 0.1 μm. The maximum deposition thickness of the Si3N4 film is typically less than 0.2 μm because of the high tensile stresses (the silicon nitride cracks when the thickness is greater than 0.2 μm). Some dry etching steps during wafer fabrication may etch away more than 0.1 μm, producing a layer of silicon nitride with a residual thickness of less than 0.1 μm. This degradation of the silicon nitride film during the wafer fabrication process reduces the ability of the layer of silicon nitride to block mobile ions from reaching the silicon oxide layer.
Accordingly, there exists a need for a material that provides proper bonding strength between the cap and the pressure sensing element, and protects Si3N4 film from over-etching during the fabrication process.